R.S. Deelder

Retention behaviour of carboxylic acids in reversed-phase column liquid chromatography

Abstract The retention behaviour of carboxylic acids in reversed-phase and reversed--phase ion-pair choromatography on hydrophobic alkyl-modified silica gel was investigated. In reversed-phase chromatography, the influence of ionic equilibria on the distribution processes was studied. Thermodynamically valid pH and p K values were used to interpret the ionic equilibria in aqueous-organic mixtures. Special attention was paid to the effect of added indifferent salts and a model describing the influence of foreign cations on the retentin of caroxylic ions is proposed. The retention of ionized carboxylic acids can be increased considerably by adding a long-chain aliphatica amind. This counter ions is predominantlhy adsorbed on the surface of the alkyl-modified silica gel. The retention behaviour of carboxylic acids in this form of chromatography can be explained by assuming an ion-exchange mechanism.

Post-column reactor systems in liquid chromatography

Summary The sensitivity and selectivity of standard liquid chromatographic detectors such as photometers, fluorimeters and coulometric cells can be substantially improved by coupling the exit of the chromatographic column to a chemical reaction system. The additional band broadening in these reactors should be reduced to an acceptable value by an appropriate design of the system. Three types of reactors are used: tubular reactors, packed bed reactors and gas-segmented liquid flow systems. The performances and characteristics of these reactor systems are compared and some specific instrumental problems (mixing, noise level) are discussed. Rules for optimal design are given. The choice of the reactor system depends on the speed and the complexity of the reaction involved. This is illustrated by a number of practical applications of both simple, fast reactions and slower reactions that consist of several steps. Results on band broadening in the reactors and on detection limits are given. Some systems are suitable for the determination of sub-nanogram amounts.

Retension mechanism in reversed-phase ion-pair chromatography of amines and amino acids on bonded phases

Abstract The rentention behaviour of a series of amines and amino acids was studied in systems consisting of a hydrophobic column packing and water or water−propanol mixtures containing low concentrations of alkylsulphonates of increasing chain length as the mobile phase. In these system an ion-exchange mechanism governs retention. The amphiphilic sulphonate ions are adsorbed on to the hydrophobic surface and counter ions from the aqueous buffe solution are concentrated near the surface. Protonated amines and amino acids exchange with these counter ions.

Study on the retention of amines in reversed-phase ion-pair chromatography on bonded phases

Abstract The retention behaviour of a series of amines was investigated in phase systems consisting of an alkyl-modified silica as column packing material and an aqueous buffer or buffer-propanol mixtures containing low concentrations of alkylsulphonates as the mobile phase. The adsorption of the alkylsulphonates on the surface of the column packing material was studied by measuring adsorption isotherms. The amphiphilic ions form monomolecular films at the support-eluent interface. Charged electrical double layers are formed, with the sulphonate ions constituting the surface charge and the counter ions forming a diffuse layer in the eluent phase. The adsorption can be described by a Langmuirian equilibrium in which allowance is made for the electrical contributions to the adsorption energy. The retention of amines can be described by an ion-exchange process, i.e., the protonated amines exchange with the counter ions concentrated near the charged surface. The molar ion-exchange selectivity constants were found to depend on the density of the ionized monolayer.

Dispersion phenomena in reactors for flow analysis

Abstract Both coiled open tubular reactors and packed-bed reactors can be used in flow analysis. Band broadening and pressure drop in these reactors are discussed. Theoretical analysis shows that packed-bed reactors are to be preferred. It is shown that for a given residence time and equal band-broadening values the pressure drop over a packed-bed reactor is lower than over a coiled open tubular reactor. Rules for optimal design are given for coiled tubular reactors and packed-bed reactors. The application of both reactors is shown for the spectrophotometric determination of phosphate with a vanadomolybdate reagent yielding a yellow colour.

Evaluation and comparison of reaction detectors

Abstract Post-column reaction detectors are becoming increasingly popular as specific, dedicated detection systems in column liquid chromatographs. Three types of reactor are currently used: open-tubular reactors, packed-bed reactors and segmented-flow systems. The performance of these reactors was studied theoretically and investigated experimentally. A comparison was made of the performance of the various reactor types for a standard reaction. It is demonstrated that for short reaction times ( ca. 1 min) the dispersion in each type of reactor can be reduced by an optimum design to a level that is compatible with the requirements set by modern liquid chromatographics. It is demonstrated that packed-bed reactors are preferable to open-tubular reactors with regard to dispersion and pressure drop. For gas segmented-flow reactors, optimum performance can be obtained only when conventional debubblers are omitted.

Determination of trace amounts of hydroperoxides by column liquid chromatography and colorimetric detection

Abstract The sensitive and selective determination of separated compounds in effluents from liquid chromatographic columns can be carried out by continuously adding a suitable colorimetric agent to the column effluent and continuously monitoring the absorbance of the reaction mixture. However, a considerable amount of additional broadening of the chromatographic peaks may occur in these systems, especially if slower reactions make it necessary for longer residence times to be used in the reactors. It is shown how this additional broadening can be reduced to an acceptable level by using packed reactors. Some general characteristics of these reactors are discussed and rules for the optimal design are given. A method is described for the determination of hydroperoxides in reaction mixtures from the oxidation of hydrocarbons. Separations are carried out by adsorption chromatography. In an on-line packed reactor, iodine is formed by hydroperoxides in an acidic solution of sodium iodide, and the absorbance of the reaction mixture is measured at 362 nm. A delay time of 1.5 min at a temperature of 70° in the reactor involved a standard deviation of the residence time distribution of less than 1 sec. Nanogram amounts of hydroperoxides can be determined by means of this colorimetric detector.

Determination of urea and ammonia using ion-pair liquid chromatography with on-line post-column derivatization in an enzymatic solid-phase reactor

Abstract A quantitative method of analysis for urea and ammonia is described, which is based on ion-pair high-performance liquid chromatography with on-line post-column derivatization on immobilized urease. In the urease solid-phase reactor, urea is quantitatively converted into ammonia, which reacts with o-phthalaldehyde and is detected by fluorescence monitoring. The method is sensitive, highly specific and easy to use, and has a linear range of ca. 1.5 orders of magnitude for urea and ca. 2.5 orders of magnitude for ammonia. Detection limits are 0.4 ng (3·10−7 M) and 0.3 ng (9·10−7 M) for urea and ammonia, respectively. The method is used for the determination of the urea and ammonia content of samples from an urea plant and in wastewater samples.

Study on the efficiency of support-coated open-tubular columns for steroid analysis

Glass support-coated open-tubular columns for the analysis of steroids in biological samples were prepared with varous support materials. The reproducibility of the procedure of preparing the column was investigated and some fundamental chromatographic properties were studied.

Preparation and application of porous packing materials for liquid—liquid chromatography

Abstract A method is described for preparing reasonable quantities of porous supports, in size fractions with a mean particle diameter of from 5 to 30 μ, from an inexpensive starting material. Several columns filled with these materials have been examined in order to determine some of their characteristics which are of importance in liquid—liquid chromatography. The columns have a relatively high permeability and a small plate height. The material gives fast separations at low pressure gradients.